1. Field of the Invention
The invention generally relates to a lens and more particularly, the invention relates to a projection lens.
2. Description of Related Art
Due to advancements in display technologies, a new generation of displays such as the liquid crystal display (LCD), the plasma display panel (PDP), and the projection apparatus have gradually replaced the traditional cathode ray tube (CRT). Among these, the LCD, the PDP and other panel displays have shared a large percentage in residential market because of their lower thickness. On the other hand, the projection apparatus has maintained a stable market share because of its ability to provide a very large image (i.e. over 52 inches) at a comparatively low cost. Furthermore, the expansive display provided by the projection apparatus makes group viewings enjoyable, and therefore a major reason why the projection apparatus has been irreplaceable is its applications in conference meetings, report presentations, and lectures. In recent years, the projection apparatus has also become an indispensable part of the home theater.
In the projection apparatus, images presented on the light valve are small, defined images. Some examples of the light valve include the liquid-crystal-on-silicon panel (LCOS panel) or the digital micro-mirror device (DMD). A projection lens is used to project small images on the light valve onto the viewing screen to form magnified images. Additionally, in order to project larger images within a shorter distance, some conventional techniques use lenses having curved reflective mirrors in front.
Curved reflective mirrors may be categorized into convex reflective mirrors and concave reflective mirrors, wherein convex reflective mirrors usually have a larger size so that dust protecting is difficult to design for the lenses. Lenses having concave reflective mirrors in front have been disclosed in disclosures such as U.S. Patent Application Publication Nos. 20070184368 (also Taiwan Patent No. 1289210), 20080079915, and 20080192208, as well as U.S. Pat. Nos. 7,048,388, 7,441,908, and 7,123,420. Among these disclosures, U.S. Application Publication Nos. 20070184368 and 20080192208, along with U.S. Pat. Nos. 7,048,388 and 7,441,908 have suggested forming intermediate images between the lens groups and the concave projection lenses.